A fuel cell has been proposed as a clean, efficient and environmentally responsible energy source for electric vehicles and various other applications. In particular, the fuel cell has been identified as a potential alternative for the traditional internal-combustion engine used in modern vehicles.
One type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell typically includes three basic components: a cathode, an anode, and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form a membrane-electrolyte-assembly (MEA). The MEA is often disposed between porous diffusion media (DM) which facilitate a delivery of gaseous reactants, typically hydrogen and oxygen from air, for an electrochemical fuel cell reaction.
Individual fuel cells can be stacked together in series to form a fuel cell stack. The fuel cell stack is capable of supplying a quantity of electricity sufficient to provide power to a vehicle. In a vehicle power system employing the fuel cell stack, hydrogen gas is supplied to the anodes from a hydrogen storage source, such as a pressurized hydrogen tank. The air is supplied to the cathodes by an air compressor unit. In a non-hybrid fuel cell vehicle or a hybrid vehicle with an inoperable high-voltage battery, a low-voltage battery is typically employed to power vehicle components and the air compressor unit prior to operation of the fuel cell stack. In a hybrid fuel cell vehicle, a high-voltage hybrid battery adapted to store electrical energy from previous vehicle operation may also be used as a source of electrical energy prior to fuel cell stack operability. Start-up with the low-voltage battery is also generally necessary with fuel cell vehicles in freezing conditions.
During start-up of the fuel cell system, hydrogen gas is used to purge the anodes of air accumulated during shut-down. The purge is desirably rapid to minimize the known carbon degradation that occurs as the hydrogen-air front moves across the anodes. Air is also bypassed to an exhaust of a fuel cell stack during start-up to dilute exhausted purge hydrogen. Vehicle emissions standards generally require the exhausted hydrogen concentration to be less than four percent (4%) by volume. However, due to the inconsistent conditions of the fuel cell system following a shut-down period, including battery state of charge (SOC) and a variable quantity of accumulated air on the anodes, known fuel cell systems are not particularly effective in optimizing hydrogen emissions and minimizing carbon corrosion during start-up.
There is a continuing need for a fuel cell system and method that provides an efficient start-up while meeting emissions and fuel cell performance requirements, for example, under freezing conditions. Desirably, the fuel cell system and method provides a robust system start-up with minimal voltage instability, and minimizes stack degradation by optimizing the hydrogen-air front time during the start-up.